1. Field of the Invention
The present invention relates to a method and apparatus for inspecting plated-through holes in printed circuit boards, more particularly, to a new and improved method and apparatus for inspecting plated through holes in circuit boards wherein a light source is used to aid in the inspection process.
2. Description of the Prior Art
A through-hole plating method is commonly used for connecting a surface conductor to a back side conductor of a double-sided board and for interlayer connecting a multilayer printed circuit board.
The through-hole plating method comprises the steps of forming a through hole in conductor-laminated insulating substrates and chemically or electrically forming a plated layer on the inner wall of the through hole to connect the laminated conductors to each other.
Since the plated layer generally has a thickness on the .mu.m order, the plated layer of the inner wall of the through hole often suffers from pinholes, cracks, or other defects. Too large a defective portion of the plated layer results in poor electrical connection of the conductors and deteriorates the reliability of the printed circuit boards. Therefore, inspection of the through holes is required.
The inspection of plated through holes in printed circuit boards has, as a general practice, been effected either manually or electrically. Manual inspection is accomplished through the utiliization of a microscope. This, however, requires individual inspection of each through hole, which is both time consuming and very fatiguing to the inspector. The smaller the through-hole diameter, the greater the time consumed and the greater the fatigue.
Electrical inspection is accomplished, for example, through the method and apparatus for inspecting plated through holes as described in U.S. Pat. No. 3,698,821, granted on Oct. 17, 1972. In the above U.S. patent, there is provided a method and apparatus utilizing a light source for inspecting plated through holes in printed circuit boards comprising means for introducing light from the source into the interior of the printed circuit board and preventing light from directly entering the plated through holes in the board. Also provided is means for detecting the presence of light within a plated through hole in the board, the detected light entering the hole through a defectively plated wall of the hole from the interior of the printed circuit board.
The method and apparatus of U.S. Pat. No. 3,698,821 enables reliable inspection of defective portions of a through hole, however, in this invention the through holes cannot be easily located at the inspecting position. Only when the location of the through hole is found, can the method and apparatus be efficiently used.